This section is intended to introduce the reader to various aspects of the art, which may be related to various of the aspects of the present invention that are described and/or claimed below. This discussion is considered to be worthwhile as it provides background information that will make it easier for the reader to understand the different aspects of the present invention. Thus, it will be understood that these statements must be read in this light and that they are not intended to distinguish the prior art.
Any ophthalmic lens, intended to be fitted in a frame, is associated with a prescription. The ophthalmic prescription may comprise a prescription for a positive or negative power and a prescription for astigmatism. These prescriptions correspond to corrections to be made to the eyes of the wearer of the lenses to correct for defects in his vision. A lens is fitted in the frame depending on the position of the eyes of the wearer relative to the frame.
In the simplest cases, the prescription only prescribes a positive or negative power and optionally an astigmatism, and the lens is then said to be “unifocal”. When the prescription does not comprise an astigmatism, the lens is axisymmetric. It is simply fitted in the frame so that the main direction of the gaze of the wearer coincides with the axis of symmetry of the lens. For presbyopic wearers, the value of the power correction is different for far vision and near vision, because of difficulties with accommodation in near vision. The prescription is then composed of a far-vision power value and an addition (or power progression) representative of the power increment between far-vision and near-vision points; this boils down to a power prescription for far vision and a power prescription for near vision. Progressive multifocal lenses are lenses suitable for presbyopic wearers; these lenses are for example described in EP 2 249 195 B1 or EP 2 251 733 B1. Progressive multifocal ophthalmic lenses comprise a far-vision zone, a near-vision zone, an intermediate-vision zone and a main progression meridian passing through these three zones. These lenses are non-specific, in that they are tailored to the various actual needs of the wearer. Families of progressive multifocal lenses are defined, each lens of a family being characterized by an addition, which corresponds to the variation in power between the far-vision zone and the near-vision zone. More precisely, the addition, denoted A, corresponds to the variation in power on the meridian between an FV point in the far-vision zone and an NV point in the near-vision zone, these points being referred to as the far-vision reference point and the near-vision reference point, respectively, and representing the points of intersection of the gaze of the wearer and the surface of the lens when he is gazing to infinity and reading, respectively.
Conventionally, a lens may be defined by its base (or average far-vision sphere of its front face) and by a power addition in the case of a multifocal lens. From semi-finished lenses, only one face of which has been shaped with a given addition/base pair, it is possible to prepare lenses tailored to each wearer by simply machining a prescription face that is generally spherical or toroidal.
For any ophthalmic lens, the laws of optics explain the appearance of optical defects as the light rays get further from the central axis of the lens. These known defects, which comprise inter alio power defects and astigmatism defects, may be referred to generically as “ray obliqueness defects” Those skilled in the art know how to compensate partially for these defects. For example, EP-A-0 990 939 proposes a method for defining by optimization an ophthalmic lens for a wearer having an astigmatism prescription. Obliqueness defects have also been identified for progressive multifocal lenses. For example, WO-A-98 12590 describes a method for defining by optimization a set of multifocal ophthalmic lenses.
An ophthalmic lens comprises a central “optically useful” zone that may extend over the entirety of the lens. The expression “optically useful zone” is understood to mean a zone in which curvature and astigmatism defects have been minimized in order to obtain a satisfactory visual comfort for the wearer. In a progressive lens, the central optically useful zone covers at least the far-vision zone, the near-vision zone and the zone of progression.
Generally, the optically useful zone covers the entirety of the lens, which has a diameter of limited value. However, in certain cases, a “peripheral” zone is provided on the perimeter of the ophthalmic lens. This zone is said to be “peripheral” because it does not meet the conditions of the prescribed optical correction and contains large obliqueness defects. Provision must thus be made for a joint between the central optically useful zone and the peripheral zone.
There are mainly two situations in which an ophthalmic lens comprises such a peripheral zone. On the one hand, when the lens has a large diameter, which may be required by the shape of the frame, for example an elongate frame with a high face-form angle, and on the other hand, when the power prescription is high, the glass then having a large edge thickness or a large center thickness, which it would be desirable to decrease.
In the case of an ophthalmic lens intended to be tailored to a wrap-around frame, for example having a face-form angle of 15°, the lens has a front face with a high camber (i.e. a high base) of between 6 diopters (denoted “D” below) and 10 diopters, and a back face specifically calculated to achieve optimal correction of the ametropia of the wearer at the optical center and in his field of vision. For example, for a front face having a given curvature, the back face is machined to ensure the correction depending on the ametropia of each wearer.
The high camber of the front face means that the lens is very thick at its edges in the case of a lens providing a negative power or is very thick at the center of the lens in the case of a lens providing a positive power. These large thicknesses increase lens weight, thereby decreasing wearer comfort and making them unattractive. In addition, for certain frames, edge thickness must be controlled in order to allow the glass to be fitted in the frame.
For negative lenses, edge thickness may be decreased by grinding using a manual faceting machine. The lens may also be controllably thinned by optical optimization. An asphericity or an atoricity may be calculated, for at least one of the faces of the lens, this asphericity or atoricity making allowance for the wear conditions of the lens relative to those of a low-camber lens of the same prescription, in order to allow the center and edge thicknesses of the highly cambered lens to be decreased.
Solutions employing such optical asphericities or atoricities have for example been described in documents U.S. Pat. Nos. 6,698,884, 6,454,408, 6,334,681, 6,364,481 or even WO-A-97 35224.
Moreover, in the case of a lens with a strong prescription, the trimmed lens has a large edge thickness, on the nasal side for a positive lens (case of a hypermetropic wearer) and on the temporal side for a negative lens (case of a myopic wearer). These excessive edge thicknesses make it difficult to fit the lens in its frame and makes wearing these lenses burdensome.
EP 2 028 529 A1 describes a method for defining the two faces of an ophthalmic lens taking into account the prescription, information on the position of the lens relative to the eyes of the wearer and information on the geometry of the frame in which the lenses must be fitted.
This first solution of the prior art proposes to tailor the curvature of the front face of an ophthalmic lens in order to improve the esthetics of the fit of the lens on the frame.
WO 2008/037892 describes a method for defining an ophthalmic lens comprising an optically useful zone, a zone, referred to as the “peripheral” zone, such as described above allowing the edge and/or center thickness of the lens to be decreased, and a joining zone possessing a profile of curvature optimized for the comfort of the wearer.
This prior-art solution proposes to decrease the edge and/or center thickness of ophthalmic lenses by producing a local asphericity or atoricity in one face of the lens, the other face being known, in order to facilitate fitting into a spectacle frame. The proposed solution provides for a joint between the central optically useful zone and the thinned peripheral zone. The prescription is respected only in the optically useful zone. This solution is not acceptable for each and every wearer because of the “peripheral” region in which his vision is not corrected.
The solutions proposed in the prior art do not allow problems posed by the esthetics of the fit of an ophthalmic lens in a frame and those posed by the need to fit ophthalmic lenses in a frame to be solved simultaneously.
In addition, there is still a need for a lens that better meets the demands of wearers with an optimal optical performance while having a small thickness in order to improve the esthetic appearance of the lens and the comfort of the wearer.